Information is often transferred between two computers over public telephone lines through the use of modems at each computer. This allows the user to transfer the text of documents, once keyed into the computer, very quickly. There are some limitations with this type of data transfer. The recipient must be ready to receive the data. Only textual type of matter, such as letters, numerals and symbols, can be transmitted in this way. Graphic material, such as photographs, charts and drawings, cannot be so transmitted. Also, unless the software between the two computers is compatible, only blocks of information can be transmitted; special control characters, such as those for centering of a topic heading, are not usable at the receiving end when transmitted. Therefore the data often must be reformatted to look anything like, for example, a letter.
Electronic mail systems have become very popular of late because they free the parties from having to make sure that the other party is ready to receive the message. Basically, electronic mail systems use a specially programmed computer or network of computers through which all messages and documents pass. In practice, the user first telephones the electronic mail system to gain access to it. After the user's authority has been verified, the user transmits (uploads) the message or document for later access (download) by another subscriber to the electronic mail system. The user can also check to see if the user has any waiting messages or documents. If so, the recipient user can ask the electronic mail system to transmit (download) the message or document to the user's computer for viewing, printing or storage.
Some electronic mail systems are relatively small and are used to connect computers or terminals within an office. Another type of mail system, called a distributed electronic mail system, uses several computers at different locations. This type of system has gained popularity for users wishing to transfer data across long distances or between different physical facilities. Distributed electronic mail systems typically transfer data over long distances using high speed lines and locally using slower transmissions over the public telephone system. Concentrating long distance communication over high speed lines is important in reducing costs. As used in this application, high speed data transmission lines refers to transmission of data at speeds orders of magnitude greater than that conventionally used to transmit facsimile data over public telephone lines. For example, normal facsimile data transmission over the public telephone system is typically from 300 to 9600 bits per second while high speed data transmission commonly occurs at speeds ranging from 9600 to 256,000 bits per second.
Some types of electronic mail systems are usable by the general public so that anyone with a computer or terminal can access the electronic mail system through the public telephone system. These public access electronic mail systems can be the distributed type or they can locate all their hardware and software at one central location depending on communications costs.
Electronic mail systems have many advantages over simple computer-to-computer document transmission systems; they are still, for the most part, limited to text. Also, if one wishes to transmit the text of a message already typed or printed, this information must either be keyed into the computer before it can be transmitted or transformed by an optical character reader, which is often not possible or not practical.